Utilization of digital images has recently been increasing in the field of medical diagnostic imaging. For example, an apparatus which radiographs an X-ray image using a semiconductor sensor has an advantage in practical terms over a conventional radiographic system using a silver halide film. More specifically, the apparatus records an image over an extremely wide radiation exposure area, and it is easy to build a system efficient in image storage and transmission using the apparatus.
Digitization of medical images allows a diagnosis form which is hard to implement using a conventional silver halide film. More specifically, to compare X-ray images radiographed at different time points during, e.g., a follow-up for a patient, a conventional process of placing a film on a viewing screen and comparative radiogram interpretation is generally performed. In this interpretation, it is important to find a difference. To find a difference between two films, rich experience and power of observation, i.e., the ability to carefully observe details are necessary.
With digital images, a change between images can be grasped more accurately. More specifically, two digital images radiographed at different time points are aligned such that their normal anatomical structures coincide with each other, and a difference process is performed to generate and output a difference image. The difference image is interpreted while being compared with the pair of original images. The generated difference image indicates only a difference. Accordingly, comparative radiogram interpretation can be performed more easily than interpretation using films. This is a technique which supports radiogram interpretation called difference-over-time CAD (Computed Aided Diagnosis).
U.S. Pat. No. 5,359,513 discloses such processing method of generating a difference image. With this method, two chest X-ray images radiographed at different time points can be aligned to generate a difference image.
FIG. 16 is a diagram of the arrangement of such difference image generation display apparatus. The first and second images are medical image data of a specific part radiographed at different time points. Since the images are radiographed at the different time points, the radiographic conditions of the two images do not completely coincide with each other. For example, in the case of X-ray images, radiographic condition parameters such as X-ray dose, tube voltage, tube current, and the like do not coincide with each other. The images are different from each other in image density and different in position and posture of an object to be radiographed.
Two image data obtained by radiographing an object are corrected in a density correction unit 1 such that their image signal density value distributions are substantially equal to each other. The corrected image data are corrected in an alignment unit 2 using the positional relationship between their anatomical structures. Corresponding pixels of the image data undergo a difference process in a difference operation unit 3 to generate a difference image. The difference image is displayed on a display unit 4 together with the first and second images.
However, difference-over-time CAD is not completely satisfactory in a medical image processing method of detecting a change over time. The precision of the difference process depends on the precision of a correction process which uniformly corrects the image signal density value distributions in the density correction unit 1.
For example, two images obtained by radiographing an object from a single position are considered. Assume that the first image is radiographed at time t1 and second image is radiographed at time 2. Also, assume that the postures of the object are the same for the sake of descriptive simplicity. Although the first image and second image have many commonalities or similarities, they are always different in details and density. For example, if an object is the chest of a patient, and X-ray imaging is used as a radiographic method, the first and second images are different in diaphragm position or the like and also different in X-ray dose. Accordingly, resultant images are different in density. Even in the case of non-X-ray images, the first and second images cannot be obtained in completely the same manner when an object is a human.
Letting x1 (x,y) be the density of the first image, x2 (x,y) be the density of the second image, and Δ(x,y), be the density of a change-over-time portion, these densities have the following relationship:x1(x,y)=s(x,y)x2(x,y)=a·s(x,y)+Δ(x,y)where s(x,y) represents the density of a structure common to the first and second images, and a is a constant. Since the radiographic conditions for the first and second images are different, the constant a is not limited to 1. The density correction unit 1 estimates the constant a such that the density of the second image is equal to that of the first image. If the estimation result is a′, a difference process result h(x,y) is represented by:
                              h          ⁡                      (                          x              ,              y                        )                          =                ⁢                              x            ⁢                                                  ⁢            2            ⁢                          (                              x                ,                y                            )                                -                                                    a                ′                            ·              x                        ⁢                                                  ⁢            1            ⁢                          (                              x                ,                y                            )                                                              =                ⁢                              a            ·                          s              ⁡                              (                                  x                  ,                  y                                )                                              +                      Δ            ⁢                                                  ⁢                          (                              x                ,                y                            )                                -                                    a              ′                        ·                          s              ⁡                              (                                  x                  ,                  y                                )                                                                            =                ⁢                                            (                              a                -                                  a                  ′                                            )                        ·                          s              ⁡                              (                                  x                  ,                  y                                )                                              +                      Δ            ⁢                                                  ⁢                          (                              x                ,                y                            )                                          
If the constant a is correctly estimated, since a′=a, the following equation holds:h(x,y)=Δ(x,y)On the other hand, the estimation result is not correct, the following inequality holds:|(a−a ′)·s(x,y)|>|Δ(x,y)|The change over time Δ(x,y) cannot be detected and is buried in (a−a′)·s(x,y) on the left-hand side. If the detailed structures are not different, the change over time Δ(x,y) becomes 0, and 0 should be output as the difference process result h(x,y). If the estimation result of the constant a is not correct, the following equation holds:h(x,y)=(a−a′)·s(x,y)
In other words, even if no change such as lesion occurs in an object, the doctor may determine that there is a change over time. This determination induces erroneous diagnosis of the doctor.